Need Tach Advice!

[jrichker]

@jrichker - about the ground for the instrument cluster etc., Is there a post or bolt with a ground wire on the inside of the firewall in the same location as the outside one to the engine? Or where do they ground so the engine side ground works? The gauges all topping out under certain combinations, starting with the temp gauge, like when the tank is full and the AC is on, is something I have experienced in a Fairmont, at least two Mustangs and in an 87 Cougar. I want to be ready if my 91 5.0 starts to act this way.
Someone always ends up saying refer to the wiring diagram book I do not have. I just want to know where the wires on the dash side ground to. Can you please help???

You typed
“ Temp gauge problem: The secondary power ground is between the back of the intake manifold and the driver's side firewall. It is often missing or loose. It supplies ground for the alternator, A/C compressor clutch and other electrical accessories such as the gauges. The clue to a bad ground here is that the temp gauge goes up as you add electrical load such as heater, lights and A/C”

What i said is accurate, so keep it in mind that it is for only engine mounted gauge sensors. Those are the oil pressure sender and engine coolant temperature sensor which is mounted in the driver's side lower intake manifold. The engine mounted gauge sensors for temperature and oil pressure use a ground from where they screw into the block. The gauges use the resistance of the senders to provide a reference voltage to make the indicator work.

If you need precise locations for grounds, check out the EVTM manual for your year car. Try google to find price and availability for the specific EVTM for you car.

The stock tach uses a feed from the computer, so it is not affected by engine ground problems

 

[7991LXnSHO]

I do not think you understood my question. Or I was misinformed about how the gauges work.

What i said is accurate, so keep it in mind that it is for only engine mounted gauge sensors. Those are the oil pressure sender and engine coolant temperature sensor which is mounted in the driver's side lower intake manifold. The engine mounted gauge sensors for temperature and oil pressure use a ground from where they screw into the block. The gauges use the resistance of the senders to provide a reference voltage to make the indicator work.

If you need precise locations for grounds, check out the EVTM manual for your year car. Try google to find price and availability for the specific EVTM for you car.

The stock tach uses a feed from the computer, so it is not affected by engine ground problems

 

[jrichker]

@7991LXnSHO

Go back and re-read this entire post. If I didn't answer you problem, restate exactly what it is that you are trying to do. The tach is not affected by bad grounds in the same manner the oil pressure and temp gauges are.

 

[George7]

Did you ever get a reply to your message? I have a 91 LX 5.0 with the same problem. Any electrical load causes the temp gauge to rise, with or without the ignition switch on or with the ignition wire connector in. I have checked the grounds and have installed a secondary ground from the engine block to the firewall. The instrument cluster grounds check out OK . I am thinking there is a short to ground in the instrument cluster or Wire 39 from the sending unit to temp gauge.
The sending unit body is grounded to the block and the threated rod contact reads about 250 ohms depending upon temperature. This all started when the volt meter stopped reading, but that is fixed as it was or appeared to be, just a loose connection to the circuit board of the instrument cluster.
Any thoughts are appreciated.